This invention relates in general to a method and apparatus for cleaning air recuperators and, in particular, to an on-line method and apparatus for air-recuperator cleaning.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a method and apparatus for on-line cleaning of finely-divided carbon black powder deposits from gas-to-gas tube and shell recuperators.
In the production of carbon black or other highly dispersed, high-surface activity solids formed by pyrogenic processes, the solids are transported by a gas stream for ultimate deposition in flexible tube filters or similar devices. Before depositing the solids materal on or in such filters, it is desirable to remove the heat from the transporting gas for reuse in the system. Therefore, the heat from such transport gas is generally removed by heat exchangers which include bundles of tubes through which the transport gas and carbon black solids material is passing. As the carbon black is carried through the tubes by the transport gas, deposits of carbon black particles form on the internal walls of the tubes. These deposits reduce both the flow of the transport gas through the tubes, and the efficient transfer of heat from the gas. In particularly aggravated solutions, a complete blockage of an individual tube can occur which may lead to damage of the heat exchanger. Since the ends of the tube are secured in a tube sheet, a tube in which the carbon build-up has caused a blockage becomes cooler than the adjacent tubes through which the hot transport gas is being passed. As a result, the cooler tube produces contraction stresses on the tube sheet which can tear the blocked tube free from its mounting in the tube sheet.
In an attempt to resolve this problem, some heat exchangers are designed such that the decrease in heat transfer efficiency is attempted to be compensated for by increasing the heat exchange surface area thereby oversizing the apparatus for the needs of the process. However, such oversizing is a temporary solution to the problem. As the unit is in service, deposits and subsequent fouling will eventually occur, decreasing the system efficiency and resulting in a decrease below process requirements.
Various methods and apparatus have been utilized to clean carbon black deposits from the tube interiors such as chemical treatment with or without mechanical scrubbing. However, chemical methods require that the unit be taken off-line and out of production, as well as requiring expert handling of the chemicals to prevent damage to the metal parts of the heat exchanger. The various mechanical cleaning methods which have been utilized require a large amount of manual labor, such as by forcing a long worm or wire brush through each tube, or sandblasting.
Accordingly, different attempts have been made to prevent such excessive build-up by utilizing cleaning jet nozzles such as disclosed in U.S. Pat. Nos. 2,069,574; 3,364,983; 4,141,754; and 4,366,003. Each of the systems disclosed in these patents utilizes a discharge of high pressure air from jet nozzles positioned over an inlet opening to the tube in order to provide a short burst of a cleaning gas into the processing tubes. While such systems may be somewhat satisfactory, they increase the mechanical complexity of the overall system, and necessitate added controls and maintenance costs.